"These are the first definitive detections of carbon-dioxide snow
clouds," study researcher Paul Hayne said in
a NASA press release. "We firmly establish the clouds are
composed of carbon dioxide — flakes of Martian air — and they are
thick enough to result in snowfall accumulation at the surface."

Interestingly, this doesn't mean that Mars doesn't have water
snow — it's been detected there too, in the northern latitudes.